One provision of the Waste Management (Amendment) Bill before the Oireachtas is the long-awaited levy on plastic bags.

As a tax, this levy is unusual; its declared purpose is not to raise money but to change behaviour. Its success will be judged not by how much money it raises, but how little.

I was surprised, during last week's second stage debate in the Seanad, to hear several of my colleagues declare this levy to be unworkable. On the contrary, I believe it is eminently workable, provided the regulations governing it are carefully drawn.

At customer level, scepticism about the measure seems to focus on the fear that the levy will add a further cost to the ordinary shopping bill. Let me put that worry to rest straight away. A levy of 15p on every plastic bag used in retailing is much too big either to be absorbed by the retailer or added direc tly or indirectly to the customer's bill.

For the shop to absorb it would be to give away the profit on about £10 worth of goods, which is far more than the average bag actually contains. For the customer to pay, it would mean an immediate increase in the shopping bill of perhaps 4 per cent or 5 per cent - an imposition that in my view customers would find totally unacceptable.

I believe that, faced with this levy, neither the shop nor the customer will choose to pay it. The levy will be the stick that will, quite quickly, change our national behaviour in regard to plastic bags - with consequent benefits in regard to our litter problem.

As a retailer, I would be heartily glad to see the back of the plastic bag problem. No one enjoys being pilloried as a desecrator of the environment and there is no doubt that grocery outlets have been the prime conduit for an ever-increasing number of plastic bags - estimated at about 1.2 billion every year.

We also have a sound business reason to be on the side of the angels: plastic bags add a significant cost to shopping which could be better used in lower prices to the customer.

This is why my company and others have engaged in many initiatives over the years to encourage customers to use fewer bags and to use more eco-friendly alternatives. The reality however is that people will probably not change their practices until they have a strong financial incentive to do so.

The levy proposed in this Bill is such an incentive. On the face of it, this measure is well designed to create the effect intended by the Minister for the Environment. Having said that, the success of the entire initiative will depend on two key practicalities being addressed:

First, that all retailers are treated equally. If the working of the scheme gives a competitive advantage to one retailer over the other, that would create an intolerable situation and one which would fatally undermine the whole project.

Second, that the line is correctly drawn between bags which are liable to the levy and those which are exempt.

We must not, for instance, make the mistake of labelling all plastic bags as environmentally unfriendly. We sell our customers a thickgauge bag that is clearly intended for reuse. It is obviously going to be part of any strategy we adopt to encourage customers to use this kind of bag and bring it with them when they are shopping. It would be clearly counter-productive to put the levy on these bags.

However, this is not where the line needs to be drawn between applying the levy and exempting bags from it.

To understand this, we have to draw a distinction between bags which are used at the checkout and those packed inside the shop. Bags packed inside the shop have benefits that in my opinion justify exempting them from the levy.

For instance, there are those used for "wet fresh food" - meat, fish, bacon, delicatessen items and a few others. There is a health-food reason for putting an effective barrier between them and other goods.

As the Food Safety Authority will tell you, it is of primary importance to keep cooked and uncooked meats strictly apart to avoid cross-contamination. That is a really serious food-safety issue and the packaging plays a key role in avoiding contamination.

For other wet fresh foods, on hygiene grounds you want to put a barrier between a product that can seep blood, like fresh meat, and anything else in your shopping bag. The same applies to fish, bacon and delicatessen items such as cheese.

With dry fresh foods, there is a long-standing practice of people choosing their fruit and vegetables, weighing and pricing them, and bringing a sealed bag to the checkout. Many customers value the freedom to exercise choice in this way and would be sorry to see it go.

To be frank, changing that set-up would also cause enormous disruption for supermarkets. It would be unrealistic to expect shops to revamp totally their way of selling fresh fruit and vegetables at the same time as they were focusing on eliminating plastic bags at the checkouts.

My proposal to achieve the objectives we all desire is to impose the levy only on bags which are used to enclose goods that are already inside a package. This would exempt bags used to enclose raw goods of any kind, but it would penalise all "secondary packaging" - bags used to package things that are already packaged.

The practical reality is that if we eliminate secondary packaging we will remove most of the environmental problem caused by plastic bags.

Feargal Quinn is chairman of Superquinn and an Independent member of Seanad Eireann